Optical and EPR spectroscopic studies of mammalian and bacterial cytochrome P-450 are being continued in order to ascribe chemical structure to the enzymatically functioning forms of these molecules. For the mammalian protein, the high spin ferric form is not newly synthesized after the injection of aromatic hydrocarbon. In bacterial tryptophan oxygenase, the ferric protein can exist in three interconvertible high spin forms depending upon the presence of particular buffers, allosteric modifiers, substrates or inhibitors. The protein behaves analogously in the ferro state when ligated to NO. A protein catalyzing the oxidation of hydroxyindoles has been purified 300-fold from the gill plates of Mytilus edulus. The linear electric field effect (LEFE) in the pulsed EPR is being developed as a new biophysical tool for the study of bonding and bond polarization in non-centrosymmetric systems, such as low spin ferric heme proteins. difference in crystal field contributions are directly related to the magnitude of the LEFE.